Add logtail if=$variable filtering and update log format examples

- ipng_stats_logtail now accepts an optional if=$variable parameter
  that suppresses log lines when the variable is empty or "0",
  following the same semantics as nginx's access_log if=. The
  condition is checked before format rendering for zero overhead on
  filtered requests. Filtered requests are still counted by stats.
- Log format examples updated to include $scheme for http/https
  visibility, and renamed to ipng_stats_logtail to match production.
- Robot test added for the if= filter (19 tests, 19 pass).
- FR-8.5 added to design doc for the if= semantics.

tests/01-module/01-e2e.robot

@@ -146,6 +146,15 @@ UDP logtail
     # Tab-separated format
     Should Match Regexp    ${output}    \\t
 
+Logtail if= filter
+    [Documentation]    Requests to /notfound are suppressed from logtail by
+    ...                the if=$logtail_enabled condition, but still counted.
+    ${output} =    Docker Exec    ${SERVER}    cat /var/log/nginx/logtail-udp.log
+    Should Not Contain    ${output}    /notfound
+    # But /notfound IS in the regular access log (not filtered there).
+    ${access} =    Docker Exec    ${SERVER}    cat /var/log/nginx/access.log
+    Should Contain    ${access}    /notfound
+
 VIP in access log
     [Documentation]    $server_addr resolves to real IPs, not 0.0.0.0.
     ${output} =    Docker Exec    ${SERVER}    cat /var/log/nginx/access.log